This invention relates to a piston for the tilt and trim unit for a marine propulsion unit and more particularly to one that insures a long life particularly by preventing wear from foreign objects.
As is well known many marine propulsion systems, particularly ones having larger displacements employ hydraulically operated trim and tilt controls. These systems generally permit trim adjustment when the watercraft is in motion and tilting up out of the water for trailering or service. In addition they generally incorporate a pop up damping arrangement that permits the propulsion unit to pop up when an underwater obstacle is encountered to prevent damage and return to the trim adjusted position when it is cleared.
One such arrangement is shown in Published Japanese Application, publication number Hei 07-69289, published Mar. 14, 1995. As shown in that publication, the tilt and trim arrangement comprises a clamp bracket fixed to the watercraft hull and on which a propulsion unit is pivotally supported for the trim and tilt operation. This is accomplished by a tilt cylinder mounted with its axis extending in a generally vertical direction and capable of expanding and retracting in the axial direction. The lower end of the cylinder is pivotally supported by the clamp bracket through a lower pivot and its upper end is pivotally connected to the propulsion unit through an upper pivot. A pressurized oil control system for controlling oil delivery to accomplish the desired motion.
The tilt cylinder includes a cylinder body forming a large cylinder bore into which a large trim piston is fitted. A small cylinder bore is formed around the axis in a part of the cylinder body above the large cylinder bore with its upper end externally opened and its lower end communicating with the large cylinder bore. A cylinder tube with its upper end closed is fitted into and inserted through the small cylinder bore and connected to the large piston. A small piston is fitted into a separate cylinder bore in the cylinder tube. A piston rod is provided, which has an end extending upward from the small piston through the closure, is pivotally connected to the propulsion unit by the upper pivot.
However, when the trim cylinder is operated to expand/retract in order to adjust the trim position of the propulsion unit, the propulsion unit swings up and down as the piston repeatedly slides with respect to the inner peripheral surface of the cylinder bore through the sealing body with friction, as described above. The same occurs during repeated tilt up and down operations as well as popping up and return operations as occur when underwater obstacles are encountered and cleared. Therefore, extended and repeated use of the unit may often produce a minute foreign matter resulting from the wearing of the inner peripheral surface of the cylinder bore or the sealing body.
If this accumulated foreign matter becomes positioned between the inner peripheral surface of the cylinder bore and the sealing body sliding against the inner peripheral surface, as it frequently does, it may promote wear of engaging surfaces, thus deteriorating the seal between the inner peripheral surface of the cylinder bore and the piston. This obviously decreases the service life of the unit.
Therefore it is a principal object of the invention to provide a piston construction that improves the service life of a propulsion unit tilt and/or trim cylinder.